C. I. pigment red 57:1 is a red pigment containing calcium bis[2-(3-carboxy-2-hydroxynaphthylazo)-5-methylbenzenesulfonate] as an essential component. This C. I. pigment red 57:1 is produced by coupling a diazonium salt of 4-aminotoluene-3-sulfonic acid and 3-hydroxy-2-naphthoic acid to thereby obtain an azo dye, and then laking this azo dye with an inorganic calcium compound (See Patent References 1 and 2).
The reaction ratio in obtaining a diazonium salt of 4-aminotoluene-3-sulfonic acid is 98% or more, and is almost stoichiometrically determined regardless of agitating efficiency and the performance of an agitator. Therefore, the molar ratio of a diazonium salt of 4-aminotoluene-3-sulfonic acid and 3-hydroxy-2-naphthoic acid, which are supplied as raw materials to obtain an azo dye, can be described as 4-aminotoluene-3-sulfonic acid: 3-hydroxy-2-naphthoic acid (molar ratio). Although the stoichiometric molar ratio of the both compounds is 1:1 in the coupling reaction, in consideration of agitating efficiency, the performance of an agitator, and the reaction ratio of a coupler and a base, it has been general to supply raw materials so that the actual molar ratio of 3-hydroxy-2-naphthoic acid exceeds the stoichiometric one by 0.7 to 5 mol %.
However, the present inventors have found that the unreacted 3-hydroxy-2-naphthoic acid remains in an azo dye despite the intention even though the reaction is conducted by supplying the raw materials in the aforementioned manner. In C. I. pigment red 57:1 obtained by laking without knowing the remaining 3-hydroxy-2-naphthoic acid, 3-hydroxy-2-naphthoic acid and metal salts thereof are naturally contaminated as impurities at relatively high concentrations, and these impurities are hard to be removed by washing, etc. Moreover, 3-hydroxy-2-naphthoic acid is produced when metal salts of 3-hydroxy-2-naphthoic acid are contacted with a strong acid for some reasons.
Recently, the 3-hydroxy-2-naphthoic acid have been recognized as a chemical compound having mutagenicity, and for example, a guideline for preventing a health problem on exposure to it has been published in the work in which a customer, etc. deals with it. Accordingly, the extinction or the concentration reduction of free 3-hydroxy-2-naphthoic acid begins to be expected. In specific, it is desired that 3-hydroxy-2-naphthoic acid and salts thereof are not eluted to a waste solution, for example in the case where a base ink for a printing ink is prepared by flushing of a pigment press-cake of C. I. pigment red 57:1 with a varnish.
In a large-scale production of a pigment itself, from the viewpoint of the security of working environment, it is desired that 3-hydroxy-2-naphthoic acid and salts thereof are not flowed out from C. I. pigment red 57:1 as a waste solution when raw materials are supplied, when an azo dye is produced by the coupling reaction as an intermediate, and when an effluent treatment is conducted in the stage before being provided for a customer.
[Patent Reference 1]
Japanese Unexamined Patent Application, First Publication No. 2002-338841
[Patent Reference 2]
Japanese Unexamined Patent Application, First Publication No. 2006-63306